1. Field of the Invention
This invention relates to an image fixing device to be used with a recording apparatus such as electrophotographic apparatus, electrostatic recording apparatus and others, and more particularly to an image fixing device in which a rotatable member is used for heating treatment, pressure treatment, or heating and pressure treatment for fixing a material to be fixed such as unfixed images onto a recording material.
2. Description of the Prior Art
In an image fixing device for an image forming apparatus in which unfixed images are fixed on plain paper, use has been frequently made of a system in which a fixing roller and a heating roller grip and transport a recording material such as plain paper.
Generally speaking, when a recording material (e.g. paper) supporting electrically an image formed with toner charged to positive or negative polarity is to be subjected to contact fixing, there will occur an off-set phenomenon, in which toner particles for forming toner images are deposited on a rotatable member such as roller or belt. In the prior art, for prevention of this phenomenon, a parting layer (comprising tetrafluoroethylene or silicone rubber) or a parting liquid (e.g. silicone oil) was formed on the surface of a rotatable member, but its preventing effect was insufficient. In addition to this, proposals for improvement are disclosed in Japanese Laid-open Patent Application Publication No. 55374/1980 (U.K. Patent Application GB 2035901 A) and Japanese Laid-open Patent Application Publication No. 96970/1980. According to one of these methods, a bias voltage of the same polarity as that of toner is applied externally on the rotatable member contacted with the toner image, while according to the other method, a bias voltage of the opposite polarity to that of toner is applied on the roller on the opposide side of the recording material bearing the toner image. Whereas, when the bias voltage is applied by means of a corona charger such as Corotoron, the device is made not only greater in scale and more complicated resulting in increased costs, but also arc discharging or leak will tend to occur when the corona charger is contaminated. Thus, such a corona charger was poor in reliability and safety. On the other hand, application by means of a bias roll can give only a practically small effect, and the bias roll was also impractically susceptible to contamination.
Further, when fixing is performed by pressure contacting a roller under a high pressure and at a relatively high speed, no solution of the problems is possible according to the method of the prior art, but such an operation will increase the force for permitting the toner to be offset. Such a force is liable to cause a complicated change in electrical field. Accordingly, the effect of preventing offset will become unstable. The device with such a great consumption of high voltage power requires excessive power, within a limited range of power, in addition to the power already required for fixing driving, exposure lamp, etc. as well as for heaters, and therefore a number of restrictions cannot but be imposed on the other devices.
Moreover, in the device of the prior art, not only the offset preventing effect was unstable, but also the toner image may sometimes suffer from scattering of toner which results in a disturbed image. Accordingly, when secondary copying (a copy of a copy of an original) was repeated for several times, the images frequently deteriorated.